Page 30 - Optofluidics Fundamentals, Devices, and Applications

Basic Micr ofluidic and Soft Lithographic Techniques 11

polar groups form covalent –O-Si-O-bonds with oxidized PDMS; the
channel is therefore sealed irreversibly. It should be noted that the
two surfaces must be brought into contact quickly (< 1 min) after oxi-
dation, because the surface of the oxidized PDMS reconstructs in air.
Empirical evidence shows that sealing works best when the samples
and chamber are clean, the samples are dry, the surfaces are smooth
(on the micron scale), and the oxidized surfaces are not mechanically
o
stressed. Heating a weak seal at 70 C can sometimes improve the
strength of the seal [19]. Another way to seal two pieces of PDMS
irreversibly involves adding an excess of the monomer to one surface
and an excess of the curing agent to the other. When the two surfaces
are cured together, an irreversible seal that is indistinguishable from
the bulk properties of PDMS forms [24].
Reversible Sealing
Another advantage of PDMS over glass, silicon, and hard plastics is
that it makes reversible conformal contact (van der Waals contact) to
smooth surfaces. PDMS devices can therefore be demountable, and
resealing can occur multiple times without degradation in the PDMS.
Microfluidic devices that are demountable can be used to pattern
surfaces with proteins, cells, and other biomolecules using fluid flow
[24]. Our group [26] and others [27] have performed binding assays
using a demountable device. Antibodies were first patterned on a
glass substrate by flowing a solution of antibody through a set of par-
allel channels. The PDMS device was then peeled off from the glass
substrate, rinsed, and placed perpendicular to the first set of chan-
nels. Solutions containing antigens were then introduced through the
channels. Antibody-antigen complexes were subsequently detected
at the crossings of stripes of antibodies and the channels.
PDMS channels can also seal reversibly to silicone (or cellophane)
adhesive tapes [19]. To make a mechanically stable support, double-
sided tape—with one side applied to a flat plastic or glass slab—is a
valuable component. Polymeric adhesive tapes are convenient
because they are mechanically flexible, and they form a stronger (but
still reversible) bond than that between PDMS and other flat surfaces.
They also allow nonsealing functional layers such as filter papers and
membranes to be incorporated into the microfluidic system [26].

Compatibility with Solvents
PDMS is compatible with water, and most polar organic solvents
(such as methanol and glycerol); it swells, however, in nonpolar
organic solvents (such as pentane and chloroform) [28], and will
absorb nonpolar solutes from aqueous solutions. To reduce the
absorption of small molecules and the swelling by nonpolar
organic solvents, one can modify PDMS with silica particles [29],
or coat the surface with a glass-like layer using sol-gel chemistry
[25] (Fig. 2-1).